The present invention relates to boating, and more specifically to a trim tab made of resilient material affixed in a cantilever fashion at the stem of a boat and its application to a surface piercing drive propulsion system.
Trim tabs have been used to change the xe2x80x9cattitudexe2x80x9d of a boat. Attitude is the angle of the boat relative to the water surface, and changes under different operating conditions. The attitude of a boat relative to the water surface has a profound effect on the speed and efficiency of the boat. Attitude is usually discussed in terms of the nose up/nose down adjustment of the boat, and is sometimes called the trim angle. The term xe2x80x9ctrim anglexe2x80x9d often leads to ambiguity as to whether it is the angle of the boat, or the tabs, or the outdrive being discussed. The present disclosure attempts to be more specific in discussing trim angle.
Trim tabs are usually fastened to the boat at or near the stem and on the transom or on the bottom of the hull. The transom generally forms the rearmost portion of the stem, such as the generally flat and vertical rearward end of the chassis of the boat. When underway, water rushes under the boat, causing the rear of the boat to be deflected up or down by the trim tabs. Pushing the trim tab down deflects the departing water downward to boost the rear of the boat up into the air slightly, thus bringing the bow of the boat down. Pulling the trim tab up is intended to pull the rear of the boat down and to bring the bow up. Thus, xe2x80x9cdown on the tabsxe2x80x9d means xe2x80x9cdown on the bowxe2x80x9d and, conversely, xe2x80x9cup on the tabsxe2x80x9d means xe2x80x9cup on the bowxe2x80x9d. It is noted, however, that the xe2x80x9cup on the tabsxe2x80x9d operation of prior art trim tabs has limited effectiveness. Attitude may sometimes be discussed in terms of the left or right lean of a boat under way. Leaning may be due to propeller torque, uneven weight distribution, or comering. Trim tabs may also be used correct this leaning.
Although trim tabs are an appurtenance to the hull, they serve to modify the shape of the planing surface and, therefore, from the perspective of hydrodynamics of the boat planing on the water, it is immaterial whether the tabs are considered as part of the hull or an appurtenance.
Prior art trim tabs are only somewhat effective in changing the attitude of the boat. Early prior art trim tabs were hinged where they joined the hull of the boat and usually were a rigid flat plate essentially parallel to the bottom surface of the hull. This flat plate could swing up or down several degrees via mechanical means. A major deficiency of flat plate hinged tabs is that the up tab position causes an abrupt change in the contour of the surface running on the water. This abrupt change causes flow separation at the hinge point. As with any airfoil, flow separation causes loss of lift. The hinged flat plate is simply a crude airfoil with poor lift to drag ratio and is not very successful at raising the nose of the boat. Hinge type tabs in the down position will lower the bow of the boat, but have a poor lift to drag ratio and tend to impose excessive drag in order to generate an equivalent amount of lift of present invention.
More recent prior art trim tabs are of a bending flat plate type whereby the tab is a resilient plate of uniform thickness and stiffness. A flat plate is attached solidly in a cantilever fashion to the boat hull and does not use a hinged joint but rather relies on the bending of the flat plate slightly up or down to generate a somewhat better, but still deficient, approximation of an airfoil. The bending flat plate trim tabs were flexed down and up by the boat operator to add hook or rocker as desired. Hook is usually caused by a concave surface on the bottom of the boat, when viewed from below the boat, which tends to lower the bow while underway. Rocker is usually caused by a convex surface on the bottom of the boat, when viewed from: below the boat, which tends to raise the bow of the boat while underway. The bending flat plate trim tabs were slightly more effective than hinged type plates. Although somewhat superior to hinged plate designs, the bending flat plate also has excessive drag for the amount of lift generated. Bending flat plate trim tabs are somewhat better than the hinged type in that the problematic abrupt change of angle of the hinge type is softened. This curved surface method decreases the tendency of flow separation, but uses a plate of constant flexural stiffness, so that the curvature is fairly localized and diminishes as the water moves away from the area of attachment of the plate to the hull.
The attitude of a boat may also be changed by changing the thrust angle of the propeller as ordinarily found on outboard motors and on inboard/outboard propulsion systems. This function is normally controlled by a xe2x80x9ctrimxe2x80x9d switch, which is part of an electro-hydraulic system. This method of changing the attitude of the boat and lifting or depressing the stem of the boat by changing the thrust angle of the propulsion is not as efficient as optimizing the thrust vector to be essentially parallel to the direction of travel and then adjusting the hull geometry to optimize the hydrodynamic lift to drag ratio.
Surface piercing drives are well known and are usually referred to as surface drives. One common type is mounted to project out the stern of the boat so that the propeller is located at the far aft or rearward end of an extended quasi-cylindrical thrust tube. Surface drives are designed to allow only the lower half of the propeller to be in the water at design operating conditions. In other words, surface drives only operate on the surface of the water. The depth to which the propeller is actually immersed in the water at any given operating speed has a major effect on the resistive torque load imposed on the engine. Therefore, it is desirable to vary the depth of immersion as needed but to also be able to closely control this depth to something less than half of the propeller diameter under all normal operating conditions. The propeller blade immersion or depth of partial immersion of the propeller is referred to herein as xe2x80x9cbitexe2x80x9d.
Surface piercing propeller drives are known to be very efficient propulsion systems. The most versatile surface drives have a means for thrust vectoring by changing the horizontal and vertical alignment of the propeller shaft with respect to the stem of the boat. This realignment is accomplished via a universal joint where the drive shaft exits the hull near the transom and connects to the propeller shaft. Mechanical means, usually of electrohydraulic cylinders push and pull the thrust tube in a preferred horizontal sweep or a vertical sweep. The horizontal sweep controls the steering of the boat, and the vertical sweep controls the trim of the boat. The steering sweep is somewhat effective, but the vertical sweep for controlling trim is rather deficient.
Surface drives have two primary advantages. The first advantage is that they have very little drag as only the propeller and skeg are in the water. The skeg is a fin that is affixed to the underside of the drive at a location forward of the propeller and that has a leading edge which sweeps generally downward and rearward. The skeg is supposed to protect the propeller from objects in the water and conversely to protect objects from the propeller. Neither is very successful. The skeg is also intended to contribute to the steering, but because a surface drive has a steerable thrust vector, the skeg has only a minimal contribution to the steering, especially when under power.
The skeg imposes undesirable drag and also slices a groove in the water as the water approaches the propeller blades. In this manner, the skeg causes a major disruption in the homogeneous flow field of water into which the blades are progressing as the propeller screws itself into the water. Although there is a performance penalty due to the drag forces of the skeg in the water, there is an added performance penalty over and above that simple drag force due to the field disruption and aeration of the water ahead of the propeller. This is why many competitive race boats place the rudder, if necessary for steering, off to the side of the propeller so as not to interfere with the flow field in proximity of the propeller.
The second primary advantage of surface drives is that the propeller is designed to be in a ventilating mode at normal operating conditions. In contrast, most inboards and outdrives with submerged propellers use nonventilating propellers and hence they have a maximum theoretical speed limit before the propeller goes into the cavitation mode and loses its grip. A propeller designed to be nonventilating does not work well in a ventilating mode and conversely, a propeller designed to operate in the ventilating mode, which is sometimes referred to as xe2x80x9chyperventilatingxe2x80x9d mode, does not work well in a nonventilating mode. There is no maximum speed for a ventilating or hyperventilating propeller as there is for a nonventilating propeller.
Surface drives have a disadvantage of not backing up very well. This is caused by the propeller back wash impinging on the transom of the boat and nullifying the reversing thrust. In contrast, non surface drive boats have their propellers immersed deep enough to let the reverse propeller wash carry under the hull. Surface drives also have a disadvantage of presenting an exposed propeller at the surface of the water such that a passenger could fall into contact with the blades more easily than with submerged propeller type of drives.
With ordinary full immersion outdrives and outboard motors, the trim effect on the boat can be accomplished by tilting the lower unit as xe2x80x9ctrim outxe2x80x9d or xe2x80x9ctrim inxe2x80x9d, hence tilting the thrust vector up or down, to raise or lower the nose of the boat. However, as previously discussed, the vertical thrust vectoring of the propulsion system is inefficient compared to the preferred trimming of the hull through the use of trim tabs. Using the up trim on a surface drive has a very weak effect on the attitude of the boat and usually only tends to pull the propeller too far out of the water causing the engine to overspeed. Using the down trim imnmerses the propeller too far into the water causing the engine to slow down due to overload. In general, the up and down trim of a surface drive affects the bite of the propeller much more than it affects the attitude of the boat.
Controlling the bite of the propeller has the same effect as that of changing the pitch of the propeller, or changing the propeller diameter, or even changing the gear ratio of the transmission. The difficult part of controlling this bite is that the water depth on the surfacing propeller is influenced by wave action, boat bounce, comering, or speed changes. Controlling the bite is advantageous in that it is like having an infinitely adjustable transmission to match the engine performance to the propeller load. However, sporadic changes in bite are not desirable because it is hard on the drive train, it makes the boat difficult to control, and it may be dangerous to the occupants.
Another disadvantage of surface drives is that they tend to crawl sideways due to having only the lower half of the propeller in the water, which thus imparts a net sideways force on the propeller. In other words, the propeller tries to crawl the back of the boat to the side. Side crawl pulls the thrust tube to one side and if the steering wheel is released, the boat turns sharply to that side, but if the wheel is held straight ahead, the boat veers to the other side following what is known as the xe2x80x9ccrabbing anglexe2x80x9d. If the boat has two surface drives mounted on the transom in the ordinary fashion, and they are rotating in opposite directions, the side forces tend to cancel and the boat does not crawl to the side. It is important to note, however, that under normal conditions such as wave action, boat bounce, cornering, and speed changes, one propeller can dig into the water deeper than its twin and therefore cause the boat to momentarily crawl sideways. This side motion can be chronic, unpredictable, and fatiguing to the driver.
Another disadvantage of surface drives is that the boat tends to struggle to get up on plane. Because the torque output of an engine is low at low revolutions per minute (RPM) and because the propeller is fully immersed, or xe2x80x9cfloodedxe2x80x9d, at low speeds, surface drive boats often have a difficult time getting over xe2x80x9chump speedxe2x80x9d, which is that speed when the boat actually starts to plane and the drag forces tend to decrease. The previous ways to overcome this bogging down effect is to use a multi-speed transmission, or to vastly overpower the boat with a very powerful motor.
Another disadvantage with surface drive boats is they may tend to xe2x80x9cblow overxe2x80x9d at high speeds. Blow over means that the boat goes airborne, nose up, and does a complete back flip. Blow over starts with too much nose up, then the hull starts xe2x80x9ckitingxe2x80x9d, and the propeller continues to run up under the boat, and it flips over. Although other high speed boats also blow over, the surface drive may be worse in that, as the bow of the boat starts to rise up, the extended propeller is dunked under the water surface as the boat tends to rise up on its bottom surface at the stern thus adding to the blow over condition.
Prior boat designs do not address the changing bite associated with wave action, or hull bounce, or changes in speed, or the fact that the plane of water leaving the bottom of the craft when turning is not in the same plane as when the craft is traveling in a straight line. Prior designs do not address the uncontrolled bite problem. Rather, the driver is forced to make continual up and down changes on the prior art hydraulic cylinder, which is attached to the thrust tube, to try to compensate for these continually changing operating conditions.
Embodiments of the present invention include an improvement in trim tabs used alone or in combination with surface drive systems. In one embodiment, a trim tab of tapered thickness, or otherwise xe2x80x9cregressive flexural stiffnessxe2x80x9d, is rigidly secured to the transom or bottom of a boat and provided with a power mechanism, such as a trim ram, or jackscrew, or the like, to flex the rearward end of the tab up or down so that the bottom surface of the trim tabs assumes the shape of a variable and progressive curve. It is the bottom surface of the trim tab which is the operative surface and which is in contact with the water. Control of the shape of that surface determines primarily the amount and distribution of lift forces exerted on the tab by the water. Lift can be both upward and downward in this application. It is the downward xe2x80x9cliftxe2x80x9d that is the most difficult to accomplish and control, and which is one of the improvements of present invention.
As with any surface in contact with the water, these trim tabs will cause undesirable drag forces, and there are circumstances when the boat is traveling well without need for any up or down trim, that is to say, it would be best if the trim tabs disappeared. Novel means for detaching the boundary layer, and its concomitant drag losses, or replacing the usual boundary layer with a very low friction boundary layer, are disclosed. This means drag losses can be minimized.
Lift forces and drag forces are both important in determining the efficiency of the hull and propulsion system. Both forces are determined by the length, width, shape, and texture of a surface. Having a surface with high lift and high drag is usually no better than having a surface with no lift and no drag. Which force is more important depends upon, at what speed the surface operates, and its load characteristics. The optimum ratio of lift to drag is important, but the ratio will change depending upon speed and loading of the boat. Present invention optimizes the lift to drag ratio and minimizes drag forces.
The lift to drag ratio of the trim tab, and by extension, the lift to drag ratio of the hull is important. One advantage of present trim tabs disclosed herein is the improvement in the hydrodynamic lift to drag ratio of a hull while minimizing the drag forces. Although only the trim tab flexes, it performs as though it is an extension of the hull of the boat. The ratio of the lift to drag forces of the hull is optimized for varying conditions of waves, loading, and speed. Premature boundary layer separation and consequent loss of xe2x80x9cliftxe2x80x9d is prevented by carefully controlling the shape of the bottom, or operative surface of the trim tab while the tab is flexed up or down by a high force, rigid, push pull mechanism.
It is important to note that the hydrodynamic forces of the water on the underside of the boat at high speed are enormous and carry the entire weight of the boat on a relatively small area. Therefore, the shape of the operative undersurface of the trim tabs must be controllable and repeatable, without unwanted deflection, at all operating conditions including heavy wave impact and extreme comering. Therefore, the trim tab must be designed as a very sturdy member and not deflect from its intended shape as determined by the control mechanism.
The trim tab, when considered as a cantilevered structural member, has a regressive flexural stiffniess from the forward end to the rearward end and is designed to take the shape of a variable and progressive curve which is adjustable, both up and down. The regressive flexural stiffness can be attained by constructing the trim tab with a decreasing second moment of inertia, for example, with a tapered thickness, or tapered width, or machined grooves in the top surface.
The trim tab, when considered as a hydrodynamic surface, is positioned to adjust the boat attitude to match the power characteristics of the propulsion system. The progressive curve is desirable because it allows the trim tabs to impart the necessary lift on the hull while doing so at the minimum drag on the hull. In one embodiment, the curve of the operative surface of the tab is generally that of a parabola. The two hydrodynamic forces of primary interest are the up forces on the undersurface of the trim tab in reaction to the net vertical component of momentum flux which is caused by the trim tab actually water-skiing across the water surface, and the opposing down forces causes by the suction xe2x80x9cliftxe2x80x9d due to the convex undersurface of the trim tab.
Drag forces are also important. In one embodiment, fluid ejection ports are provided near the forward end of the trim tabs. The fluid ejection ports enable gas, for example air, to be ejected from the under side of the trim tab to selectively detach the boundary layer under those operating conditions when the operator desires to eliminate the viscous drag of the water on the underside of the trim tab. These ports or fluid holes are grouped together and shaped to allow close control of the boundary layer across the under surface of each trim tab for all conditions, even xe2x80x9ctab upxe2x80x9d conditions. This provides the ability of the operator to make very fine changes to the boat while at high speed, which improves flight characteristics of the boat that may, for example, determine victory or defeat in a race. These fluid ejection ports may also be used for water ejection from, for example, the engine coolant water to modify the boundary layer. These ejection ports may also be used for ejecting a polymer and water mixture to further modify the boundary layer and thus reduce the drag forces. It must be noted that detachment, and maybe even modification of the boundary layer has resulted in loss of the desirable negative lift using devices of prior art. The present invention solves this problem. However, there are circumstances when detatchment or modification of the boundary layer is a favorable concession.
The trim tab has a regressive flexural stiffness such that when the adjustment rams connected to the rearward end of the tabs push down, the tabs add hook to the bottom of the boat, and conversely, when the rams on the rearward end of the tab pull up, the tabs add rocker to the bottom of the boat.
The trim tabs can also can be adjusted to have a transverse skew which twists the tab to predispose some degree of hook on one side and some degree of rocker on the other side of the trim tab.
The regressive flexural stiffness of the trim tabs is possible through several methods. One method is to make the trim tab of a solid material of tapered thickness. Another method is to use a ribbed flat plate and cut the ribs down to a progressively shorter height from the forward end to the rearward end. The ribs may face either up or down. Another method is to machine progressive grooves into a flat plate. Another method is to make the trim tab as a group of layered thinner platelets of progressive length much like a leaf style car spring. Another method is to fabricate a corrugated plate from relatively thin material which has large corrugations to match the strikes on the bottom of the hull, yet which taper to no corrugations on the rearward end of the trim tab.
Although the corrugation method may require custom tooling for each bottom configuration of strake size and spacing, such provides for the smoothest transition from those strakes to the desired flat contour on the leaving edge of the trim tab. The corrugation method may not be necessary if the design of the bottom of the hull allows the strakes to end sufficiently far forward of the transom such that the flow field normalizes back to a flat surface compatible with a flat undersurface of the trim tab. Another way to blend the strake into the desired flat bottom geometry is to add a fairing block to the bottom of the trim tab whereby this fairing block forms a smooth transition from one contour to the other. Under any circumstance, it is important that there not be any abrupt transitions from the bottom of the hull to the underside of the trim tab which would cause the boundary layer to separate.
It is recognized that the mounting of the trim tab to the bottom surface of the hull provides an ideal cantilever situation for some boats, however, there are some bottom surfaces which are highly irregular which may make the mounting difficult. In these latter cases, the trim tab may be mounted in an alternate fashion by bolting a rigid angle member, similar to a piece of angle iron, low on the transom with the trim tab then bolted to the bottom horizontal surface of the angle member. This method provides a smooth continuous flow surface for the water flowing across the undersurface of the boat and on to the undersurface of the trim tab.
The trim tab may be provided with a tapered width from the front end to the rearward end as viewed from above. A tapered width tends to minimize the potential of the trim tab snagging on the departing sheet of water as the boat is put into a turn. A boat designed for tight turns benefits from a trim tab which has a substantial amount of tapered width. A boat designed primarily for high speed straight running benefits from a trim tab of nearly constant width, that is to say, with nearly parallel sides.
The present invention also contemplates integrating trim tabs with a surface drive propulsion system to effectively control the attitude of the boat and to control the xe2x80x9cbitexe2x80x9d of the surfacing propeller. The operation of the overall system is optimized by controlling the attitude of the hull and controlling the bite of the propeller. Trim tabs of any design may be employed in this integrated manner, although operation is optimized with the improved trim tabs described herein. Such trim tab and surface drive integration solves several of the deficiencies of prior art surface drives. One of the deficiencies of current surface drives, without present invention, is that their trim controls do not perform well, that is to say, raising the xe2x80x9cupxe2x80x9d trim mechanism only serves to pull the propeller out of the water thus causing the motor to over speed, and lowering the xe2x80x9cdownxe2x80x9d trim mechanism only serves to fully immerse the propeller and thus overload the engine, while neither serves well to change the planing attitude of the boat.
In one embodiment, the surface drive is mounted in the traditional manner to project out the back of the boat in a plane nearly parallel to the water surface. The trim tab is mounted below the thrust tube, coplanar with the boat bottom, aligned with the surfacing propeller and positioned immediately in front of the surfacing propeller. The trim tab is pulled up or pushed down, such as by power rams or the like, to adjust the hull attitude. The trailing end of the trim tab at the rearward end may be radiused to match the left and right swing arc of the thrust tube, and thus the propeller, and is located just below the rearward end of the thrust tube. The thrust tube does not have a skeg, or the skeg is removed, to allow the propeller to run in an undisturbed flow field. The surfacing propeller is located at the end of the thrust tube just beyond the end of the trim tab so that water exiting out from under the trim tab immediately encounters the propeller. The steering ram sweeps the thrust tube in a xe2x80x9chorizontalxe2x80x9d arc close to the top surface of the trim tab. A xe2x80x9cbite barxe2x80x9d is located above and substantially parallel to the rearward edge of the trim tab to provide a smooth upper rail for the thrust tube to sweep across, left or right, for steering. The thrust tube is tensioned up against the bite bar so that as the bite bar is moved up or down, the thrust tube and hence the propeller are also simultaneously moved up or down. In one embodiment, the bite bar is stiff and hinged to the rear of the boat, and pushed up or down by a hydraulic ram or jack screw, or the like, referenced essentially to the trim tab. By this arrangement, the depth of the propeller bite is controlled independently of the trim tab position, and as the trim is changed, the bite bar follows along with it to maintain the same amount of propeller bite. The bite bar may be skewed to precisely vary the amount of propeller bite as a function of the left or right steering position. The skew feature may be advantageous if needed, to tune the system to be more stable or more responsive when the propellers dig in or kick out of the water during cornering.
It is contemplated that the trim ram may alternatively be located inside the hull and work through a lever to move the trim tab up or down. It is also contemplated that the bite ram may be fastened to the hull and not to the trim ram so that a servo lever strokes the bite ram to follow any changes in the trim ram.
A boat with traditional surface drives often has difficulty getting over xe2x80x9chumpxe2x80x9d speed because the propeller is fully immersed, or xe2x80x9cfloodedxe2x80x9d, and the torque of the motor is generally low at low speeds. Placement of trim tabs of present invention at a location below and in front of the propeller inhibits water from rising up the transom at low speed and replenishing the xe2x80x9cfloodxe2x80x9d condition around the propeller. Addition of flood walls on the sides of the trim tabs and flood skirts on the bite bar mechanism further limit xe2x80x9cfloodxe2x80x9d conditions around the propeller.
The use of trim tabs to limit the bite of surfacing propellers may be employed to minimize high speed xe2x80x9cblow overxe2x80x9d, which can be a problem with conventional surface drives. This is accomplished because the bite of the propeller is limited, by design, to half depth by the trim tab even if the nose of the boat starts to go airborne in a flying motion at high speed.
The combined operation of surfacing propeller and trim tabs improves the back up characteristics of surface drives. This is accomplished by causing some of the backwash to flow cleanly under the trim tab and contribute to the rearward force while the remainder of the backwash strikes the transom and is nullified.
The integrated surface drive and trim tabs in accordance with the present invention improves the efficiency of the surfacing propeller as it cuts into the water by minimizing the disruption to the flow field of the water which strikes the propeller. This is accomplished by removing the skeg and by positioning the propeller very close to the rearward end of the trim tab. The advantage to this is that the drag force associated with pulling a skeg through the water is removed, and the flow field of water entering the propeller is as smooth and undisturbed as possible thus maximizing the efficiency of the propeller itself by providing an unperturbed flow field.
Of particular interest to dual surface drives with opposite rotation, the integrated surface drive and trim tabs in accordance with the present invention reduces yaw of the surfacing propeller and the sporadic side crawl forces by balancing these forces in opposition to each other. In particular, the bite of each propeller is closely controlled under all conditions to reduce the sporadic forces. The use of bite control and engine speed synchronizers tends to decrease the problem of unbalanced yaw forces.
Of particular interest to single surface drives which do not have a sister unit to offset the side crawl tendencies of surface drives, present invention also contemplates a pair of bias fins pointing down into the water on each side of the propeller and at a location close to the leading face of the propeller. These fins are rotatably positioned about an axis generally vertical to the departing water surface whereby they tend to instantaneously counteract both the steady state and the sporadic side crawl forces of the propeller and confine these forces to that particular outdrive as they occur. These fins have a xe2x80x9cswept backxe2x80x9d leading edge to make them xe2x80x9cweedlessxe2x80x9d and to present an area to the water that is proportional to the area swept by the propeller at varying depths of operation. Despite the taming effect of these bias fins, they do impose an added drag force penalty to the system.
In a different application, the integrated surface drive and trim tab configuration may be employed to utilize yaw as a high speed steering mechanism, which is particularly suited for drag boats which run in a straight line. In one embodiment, this is accomplished by using a single engine with dual drive counter rotating surface drives. With this arrangement, the thrust tubes do not actually sweep left or right as part of the steering, but rather, the bite for each drive is moved down or up to add more or less thrust to either side of the boat. The differential bite mode of steering changes the direction of the boat travel by small increments while traveling at high speeds. One advantage to this is that the steering is accomplished using the existing hardware in a simplified form and there is no drag from a rudder. Another advantage is that the motor can be revved up to high speed with the propellers pulled out of the water and then dropped into the water at the green light, similar to revving up a car engine and dropping the clutch in a street drag race. Low speed maneuvering is accomplished by a simple low speed, swing down, rudder which can be controlled by the driver. At high speeds, the rudder swings up out of the water to minimize drag and therefore transfers steering authority to the bite control mechanism.
Historically, in emergency situations, such as, for example, the loss of a rudder, control for steering of dual propeller boats was accomplished by slipping one clutch or slowing one engine relative to its sister. In contrast, the present invention allows drag racers the use of differential bite control of the propellers, which are spinning at the same speed, to steer the boat as the normally intended mode of operation. In this application, much of the weight of a gimble and universal joints can be eliminated if a coupling, such as a roller chain type, or gear and sleeve type, is used to connect the drive shaft to the propeller shaft. Due to their loose fit, these types of couplings have sufficient flexibility to allow the necessary up and down swing of the propeller shaft.
It is noted that present invention trim tabs can be used with any boat and motor combination, including full immersion propeller systems common to traditional outboard motors and traditional inboard/outboard boats, which are also commonly known as stem drives. However, many high performance boaters are modifying both of these drives by raising them higher on the transom such that they operate as surface drives, but as with other surface drives, the attitude of the boat is no longer effectively changed by tilting the thrust vector as was done before. Therefore, present invention contemplates combining trim tabs with the surface drive systems as noted to provide novel means for attitude control of the boat. Present invention also contemplates using bite control mechanisms, such as, for example, xe2x80x9cjackplatesxe2x80x9d, xe2x80x9cjackboxesxe2x80x9d, bite bars, and extended bite rams, to control the propeller bite in conjunction with trim tabs for attitude control.
In one embodiment, the ever present danger from the exposed propeller is decreased by the addition of a people guard which covers the blades at low speed and retracts at high speed so as not to impair the fill performance of the propeller.